Wireless communication systems, such as the 2nd Generation (2G) (otherwise referred to as Global System for Mobile (GSM) communications and the 3rd Generation (3G) of mobile telephone standards and technology, are well known. An example of such 3G standards and technology is the Universal Mobile Telecommunications System (UMTS), developed by the 3rd Generation Partnership Project (3GPP) (www.3gpp.org).
Typically, wireless communication units, or User Equipment (UE) as they are often referred to in 3G parlance, communicate with a Core Network (CN) of the 3G wireless communication system via a Radio Network Subsystem (RNS). A wireless communication system typically comprises a plurality of radio network subsystems, each radio network subsystem comprising one or more communication cells to which UEs may attach, and thereby connect to the network.
The 3rd generation of wireless communications has been developed for macro-cell mobile phone communications. Such macro cells utilise high power base stations (NodeBs in 3GPP parlance) to communicate with UEs operating within a relatively large coverage area.
Lower power (and therefore smaller coverage area) femto-cells or pico-cells are a recent development within the field of wireless cellular communication systems. Femto-cells or pico-cells (with the term femto-cell being used hereafter to encompass pico-cell or similar) are classified under local area base stations and home base stations in the 3GPP standard specifications.
Femto cells are effectively communication coverage areas supported by low power base stations (otherwise referred to as Access Points (APs) or Home NodeBs). These cells are able to be piggy-backed onto the more widely used macro-cellular network and support communications to UEs in a restricted, for example ‘in-building’, environment. Typical applications for such femto-cell APs include, by way of example, residential and commercial (e.g. office) locations, ‘hotspots’, etc, whereby an AP can be connected to a core network via, for example, the Internet using a broadband connection or the like. In this manner, femto-cells can be provided in a simple, scalable deployment in specific in-building locations, since the quality of services (voice/data) suffer due to massive attenuation of macro cell transmissions going through concrete walls or metallised glass planes in order to reach the user in-building.
In a femto cell network it is known that there may be a very large number of femto cells compared to the number of macro cells; with femto cells often residing within, or overlapping, macro cells in the same geographic area.
Voltage Controlled Temperature Compensated Crystal Oscillators (VCTCXOs) are known for generating desired (reference) operating frequencies for wireless communication units. Such crystal oscillators have been employed in UE receivers operating in macro cells, and are also considered for use in femto cells. Although VCTCXOs are inexpensive, and therefore an attractive frequency reference component for use by wireless communication unit designers, they are known to suffer from frequency drift from their quiescent operating frequency, which is dependent upon the age of, and any temperature variations affecting, the VCTCXO.
Local oscillator (LO) frequencies for the radio receiver, transmitter and the sampling clocks for baseband data converters (for example analogue-to-digital converters (ADCs) and digital-to-analogue converters (DACs)), are derived from the frequency reference generated by the crystal oscillator. Hence, this frequency drift in the crystal oscillator needs to be carefully controlled; otherwise reference frequency drift will lead to degradation of performance in many aspects of the receiver. Worse still, reference frequency drift may eventually render the receiver incapable of decoding received signals due to frequency drifting outside a receiver ‘lock’ range. Moreover, from a transmission point of view, a communication unit is not allowed to transmit 3G signals at a frequency error greater than +/−0.1 parts per million, PPM, as per the 3GPP transmitter specifications for local area base stations or +/−0.25 PPM for home base stations.
In macro cell communications, base stations, often referred to as NodeBs, are guaranteed to have high frequency stability, as they employ expensive and, hence, highly stable crystal oscillators. The maximum frequency drift specification of macro cells, according to 3rd Generation Partnership Project (3GPP) specifications, is +/−0.05 PPM. Notably, this high accuracy macro cell reference frequency compares favourably to the lower accuracy performance of femto cell VCTCXO crystal oscillators, which are typically in a region of less than +/−10 PPM.
Clearly, it is of paramount importance that a femto cell communication unit receiver is in frequency lock with the most stable, accurate transmitter that it is receiving signals from, in order to correctly decode signals. Furthermore, it is important to achieve this high frequency accuracy before the receiver baseband modem attempts to decode the received channels. A desired frequency accuracy performance before decoding would be to reduce the frequency drift down to between +/−0.1 PPM and +/−0.2 PPM. This process of reducing the frequency drift within the receiver's decoding requirements is termed ‘frequency synchronisation’.
Though the frequency accuracy required for decoding is +/−0.1 PPM, the femto cell VCTCXO crystal has to be synchronised to a much greater accuracy. This is because the macro cell to which the femto cell VCTCXO crystal synchronises has a frequency accuracy of +/−0.05 PPM as mentioned before. This leaves the VCTCXO with a remaining accuracy budget of +/−0.05 PPM, out of which +/−0.03 PPM is reserved for variation due to, for example, temperature variations. As a result, the VCTCXO frequency error has to be maintained at a frequency accuracy of within +/−0.02 PPM. This frequency accuracy requirement is termed as ‘fine frequency synchronisation’.
Existing state of the art frequency synchronisation procedures, for example those employed within UEs, directly re-tune the wireless communication unit's hardware VCTCXO crystal to iteratively correct an estimated frequency error, thereby synchronising the VCTCXO crystal to any received RF signal, since it is assumed that the received RF signal originates from a stable reference such as a macro cell. Furthermore, it is known that such frequency synchronisation procedures frequency lock to every received individual base station (e.g. every macro cell NodeB), in turn, in order to select the best frequency to synchronise its operating frequency to.
In femto cells, it is proposed that femto cell access points (APs) incorporate a downlink (DL) receiver radio sub-system, in a similar manner to a UE receiver, in order to wirelessly receive transmissions from other wireless serving communication units, such as NodeBs and other femto cell APs. It is also proposed that a femto cell AP is able to scan for, receive, and decode transmissions from base stations, including macro cells and other femto cells, in a manner that is termed Network Listen (NWL). Network Listen can operate with base stations belonging to the same network as the femto cell AP, with base stations on the same or a different frequency band as the femto cell AP, and with both 2G and 3G base stations.
However, in a typical femto cell environment, it is likely that, in addition to macro cells, there will be many other femto cells in the residential neighbourhood. Hence, it is highly probable that the femto cell's DL receiver could frequency lock with both femto cell and macro cell reference frequency signals. As a result, the accuracy of the reference frequency signals with which the receiver is frequency locked cannot be guaranteed, due to the likelihood of at least some of them originating from a femto cell, as opposed to a macro cell.
In order to obtain an accurate fine frequency estimation from such a likely combination of femto cell and macro cell reference signals, it would be necessary to filter the frequency estimate over a long period of time (typically tens or even hundreds of frames in the case of low signal-to-noise ratio conditions). Such a process would consume most of the available time during a Network Listen process for each cell. Accordingly, it is not desirable that a femto cell DL receiver synchronises to another femto cell AP, since femto cell APs will typically employ inexpensive, and therefore less accurate, VCTCXO crystals.
Thus, there exists a need for a method and apparatus for fine frequency synchronisation in a cellular communication unit, particularly one for a 3GPP femto cell, for example a communication unit using an inexpensive VCTCXO crystal oscillator in a 3GPP combined femto cell/macro cell communication network, which aims to address at least some of the shortcomings of past and present techniques and/or mechanisms.